Laccases: Biological functions, molecular structure and industrial applications

Abstract

Laccases (benzenediol:oxygen oxidoreductase, EC 1.10.3.2) are extracellular, multicopper enzymes that use molecular oxygen to oxidize various aromatic and nonaromatic compounds by a radical-catalysed reaction mechanism (Thurston, 1994). They belong to a larger group of enzymes termed the blue-multicopper oxidase family which includes the plant ascorbate oxidase, the mammalian plasma protein ceruloplasmin and bilirubin oxidase, among others. The term Laccase stems from its original identification in the exudates of the Japanese lacquer tree Rhus vernicifera described by Yoshida (1883). Just over a century later it was characterized as being a metal-containing oxidase by Bertrand (1985). Laccases have also been found in other plants, and also insects and bacteria, but are predominant in fungi. Laccase activity has been demonstrated in more than 60 fungal strains belonging to Basidiomycetes, Ascomycetes and Deuteromycetes, being documented in virtually every fungus examined for it (Gianfreda et al., 1999). Its presence in plants appears to be far more limited than in fungi. All species of family Anacardiaceae, of which the lacquer tree is member, contain laccase in the resin products and in the secreted resin (Huttermann et al., 2001). Reports on the presence of laccase in other plants are however limited to Acer pseudoplatanus, Pinus taeda, Aesculus parviflora and Populus eruamericana (Mayer and Staples, 2002), though it is believed that they are present throughout the plant kingdom. Polyphenol oxidases, perhaps laccase-like, have also been reported in insects (Parkinson et al., 2003), and there is strong evidence for the widespread distribution of laccases in prokaryotes. The first bacterial laccase to be extensively studied was from Axospirillum lipoferum and the crystal structure of a bacterial (Bacillus subtilis) laccase is now available (Enguita et al., 2003). Sequence homology analysis suggests that laccases also occur in bacteria such as Mycobacterium tuberculosis (Alexandre and Zhulin, 2000). Laccase encoding genes have been found in Gram-negative and Gram-positive bacteria, including species living in extreme habitats, e.g. in Oceanobacillus iheyensis and Aquifex aeolicus and in the archaeobacterium Pyrobaculum aerophilum (Claus, 2004, 2003). The development of metagenomics in the next few years can be expected to lead to the discovery of novel bacterial laccases from extremophile environments. © 2007 Springer.